Switch numeric overrides to starting at 17 instead of 64 (#4312)

components/datetime/src/fields/length.rs

@@ -59,6 +59,14 @@ pub enum FieldLength {
     NumericOverride(FieldNumericOverrides),
 }
 
+/// First index used for numeric overrides in compact FieldLength representation
+///
+/// Currently 17 due to decision in <https://unicode-org.atlassian.net/browse/CLDR-17217>,
+/// may become 16 if the `> 16` is updated to a ` >= 16`
+const FIRST_NUMERIC_OVERRIDE: u8 = 17;
+/// First index used for fixed size formats in compact FieldLength representation
+const FIRST_FIXED: u8 = 128;
+
 impl FieldLength {
     #[inline]
     pub(crate) fn idx(&self) -> u8 {
@@ -69,8 +77,10 @@ impl FieldLength {
             FieldLength::Wide => 4,
             FieldLength::Narrow => 5,
             FieldLength::Six => 6,
-            FieldLength::NumericOverride(o) => 64 + (*o as u8).min(63),
-            FieldLength::Fixed(p) => 128 + p.min(&127), /* truncate to at most 127 digits to avoid overflow */
+            FieldLength::NumericOverride(o) => FIRST_NUMERIC_OVERRIDE
+                .saturating_add(*o as u8)
+                .min(FIRST_FIXED - 1),
+            FieldLength::Fixed(p) => FIRST_FIXED.saturating_add(*p), /* truncate to at most 127 digits to avoid overflow */
         }
     }
 
@@ -84,13 +94,13 @@ impl FieldLength {
             5 => Self::Narrow,
             6 => Self::Six,
             idx => {
-                if idx < 64 {
+                if idx < FIRST_NUMERIC_OVERRIDE {
                     return Err(LengthError::InvalidLength);
                 }
-                if idx < 128 {
-                    Self::NumericOverride((idx - 64).try_into()?)
+                if idx < FIRST_FIXED {
+                    Self::NumericOverride((idx - FIRST_NUMERIC_OVERRIDE).try_into()?)
                 } else {
-                    Self::Fixed(idx - 128)
+                    Self::Fixed(idx - FIRST_FIXED)
                 }
             }
         })
@@ -106,7 +116,7 @@ impl FieldLength {
             FieldLength::Wide => 4,
             FieldLength::Narrow => 5,
             FieldLength::Six => 6,
-            FieldLength::NumericOverride(o) => 64 + o as usize,
+            FieldLength::NumericOverride(o) => FIRST_NUMERIC_OVERRIDE as usize + o as usize,
             FieldLength::Fixed(p) => p as usize,
         }
     }